This invention relates to an electrochromic device comprising a solid layered structure in which an electrochromic layer is sandwiched between a pair of electrodes.
Ther term "electrochromism" is employed to denote the known property of a material whereby its electromagnetic radiation absorption characteristic is altered under the influence of an electric field. Such materials, for example, may exhibit little or no absorption of visible wave lengths in a first state and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, turning blue in colour. Similar effects can be observed in other portions of the electromagnetic radiation spectrum, invisible as well as visible.
The electrochromic material may be persistent, that is to say it may be a material responsive to the application of an electric field of a given polarity to change from a first persistant state in which it is essentially non-absorptive of electromagnetic radiation in a given wave length region, to a second persistent state in which it is absorptive of electromagnetic radiation in the given wave length region, and once in said second state, is responsive to the application of an electric field of the opposite polarity to return to its first state. Certain of such materials can also be responsive to a short circuiting condition, in the absence of an electric field, so as to return to the initial state.
By "persistent" is meant the ability of the material to remain in the absorptive state to which it is changed, after removal of the electric field, as distinguished from a substantially instantaneous reversion to the initial state, as in the case of the Franz-Keldysh effect.
If a layer of a persistent electrochromic material is disposed between a pair of electrodes across which a potential is applied, the radiation transmitting characteristic of the material will change. If the electrodes and the electrochromic layer are formed on the surface of a transparent substrate, such as glass, the light transmitting characteristics of the combination can be varied by controlling the electric field produced across the electrochromic layer. Thus, if the sandwich of electrodes and electrochromic material on the substrate originally is clear, i.e. presenting substantially no decrease of the light transmitting ability of the substrate, application of a voltage between the electrodes to establish an electric field of the proper polarity changes the light absorption characteristic of the electrochromic material, turning it darker for example, thus decreasing the light transmitting ability of the entire assembly.
Known electrochromic materials are usually compounds of the transition metals, in particular tungsten and molybdenum oxides. The most commonly used solid electrochromic material is probably tungstic oxide (WO.sub.3), and the electrochromic phenomenon can be discussed with reference to that material. The coloration of WO.sub.3 results from the simuleaneous injection into it of monovalent cations M.sup.+ (where M is for example H, Li, Na or Ag) and electrons which partially transform the tungstic oxide into a coloured M-W-O complex.
The application of this principle to display panels has hitherto been made by forming an electrolytic cell having a succession of layers; electrode layer, electrochromic layer, cation furnishing electrolyte layer, electrode layer.
The most efficacious cation for use in such circumstances is the H.sup.+ cation (proton) because of its high charge/weight ratio. This is why many of the known embodiments of electrochromic devices use an aqueous acid solution, e.g. of H.sub.2 SO.sub.4 as cation source. There are problems inherent in sealing such devices to retain the acid, and there are also problems in that the acid attacks the electrochromic layer. In other known arrangements, attempts have been made to replace the liquid electrolyte layer by a solid, for example by forming the acid into a gel or by impregnating a porous material. These arrangements present less of a problem in sealing the panel, but the problem of progressive degradation of the electrochromic layer by acid attack remains.
Another proposal has been to use a structure in which the so called "electrolyte" is itself a solid. Solid electrolytes which have hitherto been used are not necessarily true electrolytes in the usual meaning of that term, that is to say, they are not necessarily substances which dissociate into ions. The cations (usually H.sup.+) which migrate from the solid electrolyte to the electrochromic layer may be formed within the solid electrolyte or they may traverse the solid electrolyte from another source towards the electrochromic material. Thus the term "solid electrolyte" as used in this specification includes cationic conductors.
Known electrochromic devices often also contain a depolarising layer located between the solid electrolyte and the electrode in order to permit reversibility. The depolarising layer conducts at the same time cations and electrons. It serves to depolarise the electrode and can also furnish cations for transfer through the solid electrolyte to the electrochromic material. In such devices, the cations (usually H.sup.+) are furnished by the electrolyte and/or by the depolarising layer and also by water which is adsorbed in the different layers of the devices. This water comes from the atmosphere in contact with the layers during their formation and throughout their lives. This is why this type of electrochromic device is subject to atmospheric humidity, in some cases to such an extent that under dry conditions, no coloration of the electrochromic material can be effected. Thus, one particular problem which has arisen with electrochromic devices is a differing response time which depends on the atmosphere in which they are present. This is clearly undesirable, and it is an object of the present invention to obviate this disadvantage.